Abstract: A locking and sealing assembly locks two window sashes to each other so that they can pivotally move together relative to a casing. The assembly locks the interior sash to the casing while disengaging the lock between the two sashes, so that the interior sash urges sealing material on it to press sealing material on the exterior sash against the casing to provide a weather seal. In a preferred embodiment, the assembly has a lock shaft with a pair of radial lugs. Catches each having a curved surface flange are mounted to the exterior sash and to the casing, and have lengths at least thrice the lug width. The shaft can be moved so that a shaft lug engages the exterior sash to hold the two sashes together. When the two sashes are moved to within the casing, the lock is moved to disengage the lug while simultaneously moving another lug to lock the interior sash to the casing.